The cortical ER (cER) plays a critical role in many important physiological processes in the body, including in nerve and muscle. Until recently, cER seemed to be either present or absent, depending on the cell type. This has led to the traditional view that cER is a static specialization found in some but not most cells. Now a variety of studies, including work from our laboratory, profoundly challenge this view. They suggest that - to the contrary - many if not most animal cells have the inherent capacity to dynamically produce cER in copious quantities upon suitable physiologic demand. This implies the existence of broadly-distributed yet still unknown core machinery that triggers cER biogenesis and turnover. We have developed the first system that permits the controlled induction of cortical ER (cER) and propose here to utilize it to dissect the sub-cellular pathways and molecular mechanisms that produce this fascinating but still obscure organelle. The system entails controlled dimerization of ER membrane proteins bearing the C-terminal peptide from a cER protein, such as yeast Ist2p and mammalian STIM1, which triggers proliferation of cER containing this protein in various mammailian cells that normally have little or no cER. Discovering this machinery, the pathways it generates, and the molecular logic involved is the long-term goal of this research grant, and will have a major impact on fundamental concepts in cell biology and related areas of neurophysiology and muscle physiology, among others.